Field of the Invention
The present invention relates to a semiconductor device of SOI structure with a circuit configuration comprising a MOS transistor.
Description of the Background Art
FIG. 11 is a cross section illustrating the structure of a NMOS transistor having a conventional SOI structure. In the figure, the SOI structure comprises a semiconductor substrate 21, a silicon oxide film 22 and a SOI layer 23, and a NMOS transistor is formed in the SOI layer 23.
Specifically, an N type source region 24 and an N type drain region 25 are selectively formed in the SOI layer 23, the region between the source region 24 and the drain region 25 in the SOI layer 23 becomes a P type body region 26, a gate oxide film 27 is formed on the surface of the body region 26 serving as a channel region, and a gate electrode 28 is formed on the gate oxide film 27.
In the NMOS transistor of the SOI structure as described, when the body region 26 is brought into a floating state, the current driving ability is increased by parasitic bipolar operation. The reason for this is as follows.
Referring to FIG. 11, hole-electron pairs are generated by impact ionization. At this time, in the NMOS transistor, the electrons are extracted by the drain, and the holes are left in the body region 26, thereby increasing the potential of the body region 26. This causes a drop in the threshold voltage of the NMOS transistor having a threshold voltage characteristic as shown in FIG. 12, thereby increasing the current driving ability of the NMOS transistor.
The same is true for PMOS transistors. That is, when hole-electron pairs are generated by impact ionization in a PMOS transistor, the holes are extracted by the drain, and the electrons are left in a body region, thereby decreasing the potential of the body region. This causes a drop in the absolute value of the threshold voltage of the PMOS transistor having a threshold voltage characteristic as shown in FIG. 12, thereby increasing the current driving ability of the PMOS transistor.
Thus the MOS transistor of SOI structure has the advantage that its current driving ability is increased by bringing the body region into a floating state.
The MOS transistor of SOI structure in which the body region is in a floating state is, however, susceptible to the influence of soft error. For example, if in the body region 26 of a MOS transistor, large numbers of hole-electron pairs are generated due to the incidence of .alpha. rays into the body region 26, large numbers of holes are to be stored in the body region 26. The NMOS transistor with large numbers of holes stored has no problems in its On-state, but causes a leakage current in its Off state, resulting in an unstable current operation.
Consequently, both merits and demerits arise when the body region of the MOS transistor of SOI structure is brought into a floating state. The body region of the MOS transistor staying in a floating state causes the problem that a leakage current is caused in its Off-state.